Electron beam apparatus



Jan. 7, 1969 C, w, HANKS ETAL ELECTRON BEAM APPARATUS Filed June 18.1965 Sheet4 l of 4- "$127 """H'Ulll Jan. 7, 1959 C, w, HANKS ET AL3,420,977

ELECTRON BEAM APPARATU-S Filed June 18, 1965 Sheet g of 4 Jan. 7, 1969C, W, HANKS ET AL *A 3,420,977

ELECTRON BEAM APPARATUS Filed June 18, 1965 Sheet 3 of 4 Jan. 7, 1969 C,W, HANKS ET AL 3,420,977

ELEcTRoN BEAM APPARATUS Filed June 18, 1965 Sheet 4 of 4 mex;

@www 95 United States atent O 3,420,977 p ELECTRON BEAM APPARATUSCharles W. Hanks, Orinda, Jack D. Merrill, Richmond, and Harold A.Peterson, Concord, Calif., assignors, by mesne assignments, to AirReduction Company, Incorporated, a corporation of New York Filed June18, 1965, Ser. No. 464,968 U.S. Cl. 219-121 Int. Cl. B23k 9/08; H05b7/18 9 Claims ABSTRACT F THE DISCLOSURE This invention relates generallyto a heating apparatus, and more particularly it relates to an improvedelectron beam gun assembly for heating a target.

Electron beam gun assemblies are useful in melting, casting, vaporizing,annealing, and other heat treatment of metals, alloys, compounds,plastics and other mate-- rials. Electron gun assemblies generallycomprise an electron source for emitting an electron beam, and suitablemeans for generating a megentic field in the path of the electron beamfor guiding and focusing the electron beam onto the surface of a targetto be heated.

The electron source is operated in a region of high vacuum, andaccordingly, the target and the gun assembly are disposed within asuitable vacuum enclosure main- -tained at a high vacuum, for example,less than one millimeter of mercury absolute. The gun assembly isgenerally disposed at a convenient location within the vacuum enlcosureand the electron source, which may be an emissive cathode, is energizedto emit an electron beam. The electron .beam is accelerated along aninitial path by a suitable accelerating anode into the magnetic ieldwhich guides the electron beam onto the surface. of the target.

Various types of gun assemblies are well known. One type of gun assemblyis adapted to be positioned above the target so as to direct a beam ofelectrons along a generally linear path onto the target. Gun assembliesof this type, where there is a direct line of sight between the electronsurface and the target, are susceptible to contamination and shortingdue to condensation of volatile materials evaporated from the target onthe surface of the electron source.

In order to provide for protection of the electron source fromcontamination by condensation of volatile materials and ionized atomsevolved from the target during heating thereof, another type of gunassembly is designed to be positioned to one side of the target so thatthere is no direct line of sight between the electron source and thetaIget.- Electron gun assemblies of this type are generally known astransverse field gun assemblies, and are disclosed in Patent No.3,132,198 and copending application Ser. No. 260,158, filed Feb. 21,1963. In a transverse iield gun assembly, the electron source directs abeam of electrons into a magnetic field that is transverse to thedirection of travel of the electron beam which causes the electron beamto be guided along a curving path onto the surface of the` target. Whenthe Mice electron source is positioned out of the line of sight of thetarget, the electron source is not directly exposed to materialsvaporized from the target, and evolved condensible materials do notreadily condense on the source. A substantial decrease in thecontamination of the electron source, and a resulting longer life of theelectron source is achieved.

However, when the electron source is positioned alongside and just belowthe surface of the target, as is generally the case when the targetiscontained within an open topped upright Crucible, the electron sourcemay be contaminated or shorted due to spalling of condensed materialsfrom the cooled surfaces and shields of the vacuum enclosure above thesource. This is particularly true when the gun assembly is employed tovaporize a material which contains a large amount of condensiblevolatile impurities. The positioning of a shield having a slit thereinbetween the electron source and the target, as shown in Patent No.3,132,198, partially solves this problem. However, in order for theshield to` serve its purpose, the slit must be of a narrow width whichrequires very close control of the gun assembly in order to direct theelectron beam through the slit. Further, the shield adds expense to theconstruction of the electron beam furnace and somewhat reduces the usesto which the furnace might be put. Volatile condensibles may alsocollect on the edges of the slit reducing the width 'of the slit, andspalls of condensate may fall through the slit onto the electron source.

Another problem encountered in electron beam heating apparatus resultsfrom the feeding of solid raw materials into a molten target heated by agun assembly. Generally, splashing and splattering of the moltenmaterial will occur when the solid raw materials are delivered onto themolten surface. When the elec-'tron source is disposed to one side andjust below the upper surface of the molten target, a significant amountof the molten material that is splashed out of the target may fall uponand contaminate the electron source.

The contaminati-on of the electron source by the build up of condensatethereon causes erratic operation of the electron gun and may causecomplete shorting of -the gun. Heretofore, in order to avoid sucherratic operation, it has been necessary to turn olf the heatingapparatus and change the electron sources after about 20 hours ofoperation when vaporizing a material containing a substantial portion ofcondensible volatile impurities.

It is Ia principal object of the present invention to provide animproved electron beam gun assembly -for heating a target. An additionalobject is to provide an electron beam gun assembly which has a long lifewhen used to vaporize materials containing substantial amounts ofvolatile condensible impurities. Another object is to provide anelectron beam gun assembly which is adapted to be positioned withrespect to the target to be heated so that the electron source will notbecome contaminated due to spalling of the condensate collected upon theinner surfaces of the electron beam furnace. A still further object isto provide an electron beam gun assembly which may be positioned withrespect to the target so that the electron source will not becomecontaminated by splashing and splattering which may occur when rawmaterials are fed onto the molten target heated by the electron gun.

These and other objects of the invention are more particularly set forthin the following detailed description and in the drawings, in which:

FIGURE l is a partial schematic pictorial view of a specific embodimentof an electron beam source assembly in accordance with the presentinvention.

FIGURE 2 is a side view of the assembly shown in FIGURE l.

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FIGURE 3 is a schematic cross-sectional representation ofone form ofelectron gun Iassembly in accordance with the present invention.

FIGURE 4 is similar to FIGURE 3 and shows a different embodi-ment of anelectron gun assembly.

FIGURE 5 is similar to FIGURES 3 land 4 and shows another embodiment ofan electron gun assembly.

FIGURE 6 is a partial schematic pictorial view of another embodiment ofan electron source assembly of the present invention.

FIGURE 7 is a vertical sectional view of the assembly shown in FIGURE 6.

FIGURE 8 is a partial schematic pictorial view of a rotating cruciblehaving spaced thereabout three electron gun assemblies constructed inaccordance with a further embodiment of the invention.

FIGURE 9 is an elevational front view of the apparatus of FIGURE 8.

FIGURE 10 is a sectional View taken along line 10-10 of FIGURE 9.

FIGURE 11 is a partial schematic-elevational view of a furtherembodiment of a source assembly of the present invention.

FIGURE 12 is a plan view of the apparatus of FIG- URE 11.

Very generally, and having reference to FIGURE 1 of the accompanyingdrawings, the present invention is directed to an electron beam gu`rr1 1v comprising a source 13 for emittingan electron beam, generallyindicated by numeral 15, and magnetic lens means 17 for generating atransverse magnetic field having a particular configuration in the pathof the electron beam 15 which field focuses the electron beam and guidesit onto the surface of a tar-get 19. In accordance With the presentinvention, the magnetic lens means 17 includes a pair of poles 23, 23afor establishing ya first magnetic field of increasing strength in theinitial path of the electron beam, and for establishing a secondmagnetic field of substantially constant strength in the path of theelectron beam leaving the first magnetic field. The field of increasingstrength comprises lines of flux which increase in curvature in thedirection of the electron source to thereby reduce the divergingtendencies of the high intensity electron beam.

In order to avoid contamination of the sourcel by spalling condensateand/or splashing and splattering of molten material, the source 13 isadapted to Ibe positioned with respect to the target 19 so that thesource 13 is completely hidden from the target 19, and so that theinitial path of the electron beam 15 diverges with respect to a line 27perpendicular to the surface of the target 19. As used herein, the termcompletely hidden is defined as the positioning of the source 13 withrespect to the surface of the target upon which the electron beam 15impinges so that spalling condensate from the interior surfaces of thevacuum furnace, and splashing and splattering molten Imaterial from thetarget are prevented from striking the source. Accordingly, some form ofimperiorate barrier or shield is positioned between the source 13 andthe target 19. Generally, the source 13 is disposed above or below the-s-urface of the target 19 in some convenient manner so that the sourceis shielded by the target and its associated support structure, or bythe structure of the gun assembly itself. Alternately, the source 13 maybe disposed to one side of the target beneath an imperforate shield 29(FIGURE 7) which positively protects the source from contamination byspalling condensate and/or -splashing molten material.

In a preferred embodiment of the invention, the source 13 and themagnetic lens means 17 are so disposed with respect to one another thatthe source 13 is located within the fringing portion of the ymagneticfield of increasing strength generated between the pole pieces 23, 23a.

When the electron gun assembly is constructed so that the source 13 willybe completely hidden from the target, the electron beam is guided overan arc of approximately or more from the source to the surface of thetarget.

Theoretically, a uniform transverse magnetic field is capable of guidingan electron beam through an arc approaching 180 or more. However, it hasbeen found that a uniform transverse magnetic field is not particularlysuited for guiding a high power density diverging electron beam, e.g.,an electron beam having apower density of 50 kilowatts per square inch,over an arc of about '180 or more, inasmuch as the diverging high powerdensity electron beam L*is not focused, i.e., converged, by the uniformtransverse magnetic field. It has been discovered that in order to guidea high kpower density electron beam through an arc approaching 180 ormore, such as when the source is completely 'hidden from the target, aparticular magnetic field configuration is desirable in order to providea magnetic lens for preventing divergence of the electron beam and forfocusing the electron beam onto the surface of the target.

Referring to FIGUREv 3, there is shown a schematicrepresentation of thesource 13 for emitting the beam of electrons 15, the target 19, and thetransverse magnetic field generated by Athe magnetic lens means 17 (notshown), in accordance with one embodiment of the present invention. Thesmall circles 31 represent the lines of fiux generated by the magneticlens means 17, and the distance between the circles illustrates therelative strength of the magnetic field. The transverse magnetic fieldcauses the l electron beam 15 generated by the source 13 to travel alonga curving path onto the surface of Ithe target 19.

As shown in FIGURE 3, the source 13 is completely hidden from the targetby being disposed beneath the target 19 in alignment therewith. Thesource 13 emits the electron beam 15 along an initial path which may begenerally designated as a linear path, although it is understood thatthe beam may be diverging and may be traveling in a slightly arcuatepath. A first magnetic field of increasing strength, generally indicatedby numeral 35, is established adjacent the source in the initial path ofthe electron beam in the area between "the source and the line indicatedby numeral 37. The magnetic field 35 of increasing strength comprisescurving ilines of iiux which bow downwardly and which have increasingcurvature in the direction of the electron source. Accordingly, thecurving lines of linx of the magnetic field 35 of increasing strengthcause the electron beam to be compressed or converged in a planegenerally perpendicular to the plane of FIGURE 3.

As further shown in FIGURE 3, a second magnetic field, generallyindicated by numeral 39, of Substantially constant strength isestablished in the path of the electron beam leaving the first magneticlield 35 of increasing strength between the line 37 and a line indicatedby numeral 41. The second magnetic field 39 of constant strength furtherdefiects the electron beam 15 toward the target 19, and converges theelectron beam in a plane parallel tothe plane of FIGURE 3.

It Ihas been determined thatin order to provide for focusing of theelectron beam 1S, the magnetic field 35 of increasing strength shouldextend over a first portion of the curving path of the electron beam,and the magrie-tic field 39 of constant strength should extend over atleast a portion of the remainder of the path of the electron beam. Asdiscussed hereinafter, when the magnetic field 39 of constant strengthextends over only an intermediate portion of the curving path of theelectron beam, a magnetic field, generally indicated by numeral 43, isestablished over the remainder of the curving path of the electron beambetwen the line 41 and the surface of the target.

The portion of the path of the electron beam 15 exposed to the magneticfield 35 of increasing strength, and the portion of the path of theelectron beam exposed to the magnetic field 39 of constant strength maybe varied. It has been generally determined that the magnetic field 35of increasing strength may extend over the first quarter to about thefirst one-'half of the curving path of the electron beam, and that themagnetic field`39 of constant strength may extend over at least anintermediate portion of the curving path of the elctron beam from theterminal edge of the magnetic field 35 of increasing strength to atleast a point about one-half to about twothirds of the distance alongthe curving path of the electron beam.

The strength of the magnetic field 43 may be constant, or may weaken,i.e., decrease, toward the surface of the target as may be desired. Themagnetic eld 43 illustrated in FIGURE 1 is progressively weaker towardthe surface of the target. It lhas been determined that a constant fieldstrength over about the last one-half to two-thirds of the curving pathof the electron beam provides for the maximum focusing and concentrationof the electron beam on the target. A schematic representation of anelectron gun having a magnetic field of this configuration isillustrated in FIGURE 4. Particularly desirable results are obtainedwhen the magnetic field/ofincreasingstrength extends overthjuirstmthiriofliheacurving path of the electron beam, and "themagnetic field 39 of constant strength extends over the remainderofthpath of the electron beam. l

Desirable results, and good focusing and concentration of the electronbeam on the surface of the target are obtained Where the magnetic field35 of increasing strength is established over a first portion of t e cupath of the electron beam, preferably over about the first onethird ofthe path, the magnetic field 39 o t strengt blisliedv over anintermediate portion of the curving path of the electron beam,preferably over about the middle one-third of the path, and the magneticfield fllhaiugonstant strength less than the strenth of the magneticfield.39,.is established ovr the remain er of the curving path of theelectron beam, preferably over the last one-third of the path. Anelectron gun assembly cluding means for establishing magnetic fieldshaving hese configurations is schematically illustrated in FIG- RE 5 ofthe drawings.;

.l For most operations, it is not necessary that the mag- ;netic fieldstrength remain constant over the terminal vportion of the curving pathof the electron beam. In fact,

in some instances, as when a rotating crucible is employed, thestructure of the target is such that it is not possible to establish amagnetic field which has a constant field strength over the terminalportion of the path of the electron beam. In such instances it has beenfound that equally desirable results may be attained when the strengthof the magnetic field weakens progressively along the terminal portionof the electron beam path as shown in FIGURE 3 of the drawings.

Referring now to FIGURES 1 and 2 of the drawings, there is shown aschematic pictorial view of a specic embodiment of an electron gunassembly 11 for focusing an electron beam. 15 onto the surface of atarget 19, which may, for example, be a pool of molten material disposedin an upright open ytopped crucible 47. The electron gun assembly 11includes the magnetic lens means 17 for generating magnetic fieldshaving the configuration illustrated in FIGURE 5,. A

The electron gun assembly 11 includes a generally horizontalferromagnetic base plate 49 which forms the return path for the magneticflux as discussed below. The base plate 49 is provided with a notch 50adjacent one edge thereof, which for convenience may be designated thefront edge. Four upstanding electromagneticmoils 51,535/and-i74or-generating a magnetic fiux as later descri e are mountedon the base plate adjacent the corners thereof and are suitablyconnected to a source of direct current (not shown).

L-shaped pole pieces 61, 61a are supported atop the coils 51 and 53, and55 and 57, respectively, so that one leg 63, 63a thereof is generallyupstanding. The upstand- 75 ing legs 63, 63a are disposed in generallyparallel relationship and are spaced apart a distance sufficient toreceive the crucible 47 containing the target material 19 therebetween.Diamagnetic support means 65 may be provided on the base plate 49 .forsupporting the crucible 47. The L-shaped pole pieces 6l, 61a are of aAsufficient length so as to extend beyond and overhang the base plate 49adjacent the front edge thereof.

Ferromagnetic support members 67, 67a are attached at right angles tothe forward'end offeach of the upstanding legs 63, 63a respectively,extending inwardly of the space defined between the L-shaped pole pieces61, 61a. The pole pieces 23, 23a, formed of a ferromagnetic materialdepend from the inward end of each of the support members 67, 67a. rPhedependingy pole pieces 23, 23a are generally parallel to one another andextend outwardly and downwardly of the L-shaped `pole pieces 61, 61a andsupport members 67, 67a to a point below the the base plate 49. As seenin FIGURE l, the depending pole pieces 23, 23a are disposed adjacent thenotch 50 in the plate 49, and are attached to support members 67, 67aover approximately one-half of their length. The lower portions of thedepending pole pieces 23, 23a that are not attached to the supportmembers 67, 67a define first pole sections 69, 69a, and the upperportions of the depending pole pieces 23, 23a, attached iniiux-conducting relationship to the support members 67, 67a, definesecond pole sections 70, 70a. A magnetic circuit is thus defined by thebase plate 49, coils 51, 53, 55 and 57, I.-shaped pole pieces 61, 61a,support plates 67, 67a and the air gap between the upstanding legs 63,63a of the L-shaped pole pieces 61, 61a and between the depending polepieces 23, 23a. The location of the depending pole pieces 23, 23a in theregion of the notch 50 prevents a magnetic circuit from beingestablished between the depending pole pieces .23, 23a and the baseplate 49.

As best seen in FIGURE 2, an electron source 13 is supported by asuitable support means (not shown) at a position `beneath the crucible47 so that the source 13 will be completely hidden from the targetmaterial 19 maintained within the crucible. The electron source 13 maybe selected from several commercially available sources. One form ofcommercially available electron source generally comprises an elongatedrod-shaped lament cathode 73 disposed within a backing electrode 75, andan accelerating anode 77. The electron source for emitting the electronbeam 1 5 from the cathode 73 may be connected to a suitable power source79 in accordance with conventional procedures.

As discussed above, the electron source 13 is positioned with respect tothe target so that it will be completely hidden therefrom. Accordingly,the source is oriented so as to emit the electron beam 15 along aninitial path which is divergent with respect to the line 27perpendicular to the surface of the target material in the crucible.

The electron source 13 is preferably disposed in a fringing magneticfield which is convex with respect to the source, that is, a magneticfield in which the lines of force bow from the poles toward the source.Accordingly, as seen in FIGURE 2, the source 13 is positioned at a pointspaced from the edges of the poles 23, 23a and is oriented with respectthereto so as to emit the electron beam 15 into the fringing fieldestablished between the lower edges of the pole sections 69, 69a.

It can be seen that the path of least resistance for the,

magnetic fiux generated by the coils 51, 53, 55 and 57 is alongtheL-shaped pole pieces 63, 63a, through the support members 67, 67a, andacross the air gap between the.

pole sections 70, 70a of the ldepending pole pieces 23, 23a.Accordingly, the magnetic field will have a maximum strength in the gapbetween the pole sections 70, 70a. The magnetic field between thedepending pole pieces 23, 23a will have the least strength adjacent thelower edge of the pole sections 69, 69a since this point is the furthestdistance from the path of least resistance between the pole pieces 70,70a. The strength of the magnetic iield between the pole sections 69,69a increases along the surface thereof to the point at which the polesections 69, 69a merge into the pole sections 70, 70a, i.e., at thepoint that the depending pole pieces 23, 23a are connected to thesupport members 67, 67a.

A magnetic field of constant strength is also established between theupstanding legs 63, 63a of the L-shaped pole pieces 61, 61a. However,since the air gap between the upstanding legs 63, 63a is greater thanthe air gap between the depending pole pieces 23, 23a, the magneticfield established between the L-shaped pole pieces 61, 61a will be oflesser strength than the magnetic iield established between t-he polesections 70, 70a. Accordingly, in the electron gun assembly depicted inFIGURES l and 2, and as schematically represented in FIGURE 5, theelectron beam 15 is emitted along an initial path that is divergent withrespect to the line 27 perpendicular to the surface of the target 19into a magnetic field 35 of increasing strength established between thepole sections 69, 69a, which magnetic eld extends over approximatelyt-he first third of the curving -path of the electron beam. The electronbeam passes out of the magnetic eld 35 of increasing strength into amagnetic eld of constant strength established between the pole sections70, 70a, which magnetic eld extends over approximately the intermediate`onethird of the curving path of the electron beam. The electron beampasses out of the magnetic eld 39 into a constant magnetic eld 43 oflesser strength established between the L-shaped pole pieces 61, 61a.The constant magnetic tield 43 extends over approximately the finalone-third of the path of the beam.

In operation, the electron source may be operated at a potential of15,000 volts to emit an electron beam having a density of one amp percm?. When an electron source operated at these conditions is placedabout six inches beneath an open topped crucible, the coils 51, 53, 55,57 may be connected to a suitable source of direct current forgenerating a constant magnetic field 39 having a strength between about50 gauss and about 100 gauss between the pole sections 70, 70a. With amagnetic field of about 70 gauss established between the pole sections 70, 70a, the magnetic tield 35 established between the pole sections69,*6N9a increas`e`f'm about 35 gauss to about 70 gauss, i.e., Ey afactor of about 2, over the length of pole sections 69, 69a, and theconstant eld between the L-sha-ped poles 61, 61a has a strength of about30 gauss. Alternately, other magnetic field strengths may be employedfor controlling Iother electron beams having other power densities.

Referring to FIGURES 6 and 7, there is shown an embodiment of anelectron gun assembly 11 in accordance with the present invention,wherein a Iuniform air gap is maintained between the pole pieces 77,77a. The electron gun generally includes a plurality of coils 79, 79a,81, 81a, 83 83a, and 85, 85a mounted in flux conduction relationship ona generally U-s-haped ferromagnetic support member 89 which forms thereturn path for the ux. The pole pieces 77, 77a are connected to thecoils and define a space therebetween for receiving a generallyupstanding open topped crucible 91 which contains the target material 19to be heated. The pole pieces 77, 77a comprise generally horizontallyextending pole sections 93, 93a extending on opposite sides of thecrucible 91 at least partially above the surface thereof, intermediatepole sections 95, 95a which extend angularly downwardly and outwardly ofthe horizontal pole sections 93, 93a, and depending pole sections 97,97a which extend downwardly from the intermediate pole sections 95, 95a.The respective pole sections may be connected together, as shown o r maybe spaced apart, as desired.

A generally horizontally extending imperforate shield 29 is disposedbetween the pole sections 97, 97a. The electron source 13 is positionedbeneath the imperforate 8 shield 29 in a manner so that it is completelyhidden from spalling condensate and/ or splashing molten material fromthe target 19, and is oriented so that the electron beam 15 is emittedinto the fringing magnetic field, which is concave toward the source 13,established between the lower edges of the pole sections 97, 97a.

rl`he coils may be connected in suitablerpairs, i.e., 79 and 79a, S1 and81a, to sources of direct current for generating magnetic lields ofdifferent strengths between the pole sections 93 and 93a, 95 and 95a,and 97 and 97a. The number of turns on the respective coils and/ or thedirect current sources may be varied in order to establish a magneticfield 35 of increasing strength in the direction of travel of theelectron beam between the depending pole sections 97, 97a, and amagnetic field 39 of substantially constant strength between theintermediate Ipole sections 95, a and horizontal pole sections 93, 93a.An electron gun assembly embodying a magnetic eld configuration of thistype is schematically represented in FIGURE 4.

The electron gun assembly illustrated in FIGURES 6 and 7 provides amagnetic field which has an increasing strength over approximately theirst one-third of the curving path of the electron beam, and a constantstrength over the remainder of the path of the electron beam, and isparticularly suitable for loperations where maximum concentration of theelectron beam on the surface of the target and large flow rates ofevaporated material are desired.

A further embodiment of the invention is illustrated in FIGURES 8 to 10of the drawings. As seen in FIGURES 8 and 9, three electron gunassemblies 11 embodying the principles of the present invention aredisposed about the open mouth of a generally horizontally extendingrotating crucible 101. The rotating crucible 101 contains a moltentarget 19 (FIGURE 10) which due to centrifugal force is urged into aslot 103 in the inside wall of the crucible 101. At rotating cruciblesimilar to that illustrated is more fully disclosed in copendingapplication Ser. No. 287,386, filed June l2, 1963. Each of the electrongun assemblies 11 includes a pair of parallel spaced apart coils 105,10Sa which are radially'disposed from the side wall of the crucible andIwhich dene a space there-"- between within which is positioned theelectron source 13. The coils 105, 105a extend slightly beyond the openmouth of the crucible (FIGURE 10) and are connected adjacent theirrearward ends to a ferromagnetic 107 which forms a ux return path.

Angularly disposed pole pieces 109, 109a are attached in ux conductingrelationship to the forward ends of the coils 105, 105a. As seen inFIGURE 9, the angular pole pieces 109, 109a depend from the ends of thecoils 105, 105:1 and extend inwardly thereof at an angle. Pole faces111, 111a are attached to the ends of the angular pole pieces 109, 109a,defining an air gap therebetween.

As seen in FIGURE 9, the angular pole pieces 109, 109a dene an air gapof decreasing width over the length thereof toward the pole faces111,111a, and the pole faces 111, 111a deline an air gap of constantwidth. The electron source 13 is mounted between the coils 105, 105aadjacent the side wall ofthe crucible 101 in a position so that thesource is completely hidden from the target, and so that the electronbeam 15 is emitted along an initial path which diverges with respect toa line 113 perpendicular to the surface ot` the target material 19within the rotating crucible.

The magnetic lens shown in FIGURES 8 to 10 establishes a magnetic fieldsimilar to that schematically represented in FIGURE 3. As best seen inFIGURE 10, the electron gun is located rearwardly of the angular polepieces 109, 109a and accordingly, the electron beam is emitted into thefringing portion of the magnetic iield established between the upperends of the angular pole pieces 109, 109a. The electron beam travelsthrough a magnetic eld 35 of increasing strength, due to the demember 9creasing air gap between the angular pole pieces 109, 1090: and thenceinto a magnetic field 39 of constant strength between the pole faces111, Illa. The electron beam then enters a magnetic field 43 ofprogressively weakening strength which is provided by the fringingportions of the magnetic field 39 established between the pole faces111, 111a.

It can be seen that when the electron gun is employed to heat a rotarycrucible such as shown in FIGURES 8 to 10, it is not possible to havepole pieces extending alongside the target, and accordingly, the eldwill be weakening in the region of the target. As pointed out above,this is not particularly undesirable and close control over thedirection of the electron beam may be achieved utilizing the apparatusillustrated in FIGURES 8 to 10.

Another embodiment of an electron gun assembly including a magnetic lens17 which provides a magnetic field configuration similar to that shownschematically in FIGURE 5, is shown in FIGURES l1 and 12. The electrongun assembly includes an electron source 121 which may be locatedbeneath an open topped crucible 123 as illustrated in FIGURE 1l or maybe located beneath an imperforate shield similar to that illustrated inFIGURE 7. A pair of pole pieces 125, 12511 is mounted adjacent to thecrucible above the electron source 121 in the initial path of theelectron beam emitted by the electron source 121. The pole pieces areconnected to suitable coils 127, 127a and a ferromagnetic return path129 is provided between the coils. The positioning of the pole pieces125, 125a' above the electron source 121 provides a fringing magneticfield in the region of the electron source, which magnetic fieldincreases in strength in the direction of travel of the electron beam. Aconstant magnetic field is provided in the region between the polepieces 125, 12Sa and a magnetic field of decreasing strength is providedin the region adjacent the surface of the target within the crucible123.

The electron gun assembly shown in FIGURES ll and 12 thereby provides amagnetic field of increasing strength over about the first one-third ofthe path of the electron beam, a magnetic field of constant strengthover the intermediate one-third of the path of the electron beam, and amagnetic field of decreasing strength over the last one-third of thepath of the electron beam. As previously described, an electron gunassembly similar to that set forth in FIGURES ll and 12 is capable ofproviding close control over the electron beam and provides a simple andconvenient apparatus for heating of a target material while insuringthat the electron source will not become contaminated due to spalling ofcontaminate from the interior of the electron furnace,r or splashing ofmolten material from the target.

It is to be understood that various other configurations of pole piecesand magnetic field generating means within the skill of the art may beemployed to achieve the desired magnetic lens. The pole piecesestablishing the vari-- ous magnetic fields of different strength neednot be connected in flux conducting relationship, and the air gapsbetween the various magnetic field generating means need not be ofdifferent widths.

Various of the features of the invention are set forth in the followingclaims.

What is claimed is:

1. An electron beam gun assembly. for heating a target comprising, asource for emitting a high power density diverging electron beam in adirection away from the target along an initial path which diverges froma line perpendicular to Ithe surface of the target, said sourcepositioned rearwardly from the surface of t-he tar-get and isolatedtherefrom, and magnetic lens means between said source and the targetfor deflecting and focusing the electron beam from its initial pathalong a curving path onto the surface of the target, said magnetic lensmeans including means outwardly spaced from the target for establishinga magnetic field of increasing strength adjacent said source in theinitial path of the electron beam, said magnetic field of increasingstrength having lines of flux of increasing curvature toward saidsource, said magnetio lens including means outwardly spaced from thetarget for establishing a magnetic field of substantially constantstrength in at least a portion of the curving path of the electron beambetween the. magnetic field of increasing strength and the target.- :Y

2. An electron beam'v gun assembly for heating a target comprising, asource for emitting a high power density diverging electrorrbeam in adirectionaway from the target along an initial path which diverges froma line perpendicular to the surface of the target, said sourcepositioned rearwardly from the surface of the target and isolatedtherefrom, and magnetic lens means between said source and the targetfor deflecting and focusing the electron beam from its initial pathalong a curving path onto the surface of the target, said magnetic lensmeans including means outwardly spaced from the target for establishinga first magnetic field of increasing strength in the direction of travelof the electron beam adjacent said source in -the initial path of theelectron beam, said first magnetic field of increasing strength havinglines of flux of increasing curvature extending in a direction that isgenerally rearwardly from the surface of the target toward said source,said magnetic lens including means outwardly spaced from the target forestablishing a second magnetic field of substantially constant strengthin at least a portion of the curving path of the electron beam betweenthe magnetic field of increasing strength and the target.

3. An electron beam gun assembly for heating a target comprising, asource for emitting a high power density diverging electron beam in adirection away from the target along an initial path which diverges froma line perpendicular to the surface of the target, said sourcepositioned rearwardly from the surface of the target and isolatedtherefrom, and magnetic lens means between said source and the targetfor deecting and focusing the electron beam from its initial path alonga curving path onto the surface of the target, said magnetic lens meansincluding means outwardly spaced from the target for establishing a rstmagnetic field adjacent said source and in the initial path of theelectron beam, and means outwardly spaced from the target forestablishing a second magnetic field in the curving path of the electronbeam between said first magnetic. field and the target, said firstmagnetic field having increasing strength inthe direction of travel ofthe electron beam and extending over about the first one-fourth to aboutone-half of the curving path of the electron beam, said second magneticfield having substantially constant strength and extending over at leasta portion of the remainder of the curving path of the electron beam,whereby an electron beam generated by the said source is guided from itsintial path along a curving path and is focused onto the surfaceof thetarget.

4. An electron beam gun assembly for heating a target comprising, asource for emitting a high power density diverging electron beam in adirection away from the target along an initial path which diverges froma line perpendicular to the surface of the target, said sourcepositioned rearwardly from the surface of the target and isolatedtherefrom, and magnetic lens means between said source and the targetfor defiecting and focusing the electron beam from its initial pathalong a curving path onto the surface of the target, said magnetic lensmeans including means outwardly spaced from the target for establising afirst magnetic field of increasing strength in the direction of travelof the electron beam adjacent said source and in the initial path of theelectron beam, and means outwardly spaced from the target forestablishing a second magnetic field of substantially constant strengthin the curving path of the electron beam between said first magneticfield and the target, said first magnetic field having lines offiux ofincreasing curvature extending in a direction generally rearwardly fromthe surface of the target, said first magnetic field extending overabout the first 'one-fourth to about one-half of the distance of thecurving path of the electron beam, said second magnetic field extendingbetween said first magnetic field and a point between about one-half toabout twothirds of the distance of the curving path of the electronbeam, whereby an electron beam generated by said source is guided fromits initial path along a curving path and is focused onto the surfaceofthe target.

5. An electron beam gun assembly for heating a target comprising, asource for emitting a high power density diverging electron beam in adirection away from the target along an initial path which diverges froma line perpendicular to the surface of the target, said sourcepositioned rearwardly from the surface of the target and isolatedtherefrom, and magnetic lens means between said source and the targetfor deflecting and focusing the electron beam from its initial pathalong a curving path onto the surface of the target, said magnetic lensmeans including means outwardly spaced from the target for establishinga first magnetic field adjacent said source and in the initial path ofthe electron beam, and means outwardly spaced from the target forestablishing a second magnetic field in the curving path of the electronbeam between said first magnetic field and the target said firstmagnetic field having increasing strength in the direction of travel ofthe electron beam and extending over about the first one-fourth to aboutone-half of the curving path of the electron beam, said second magneticeld having substantially constant strength and extending over theremainder of the curving path of the electron beam, whereby an electronbeam generated by said source is guided from its initial path along acurving path and is focused onto the surface of the target.

6. An electron beam gun assembly for heating a target comprising, asource for emitting a high power density diverging electron beam in adirection. away from the target alongthe initial path which divergesfrom a line perpendicular to the surface of the target, said sourcepositioned rearwardly from the surface of the target and isolatedtherefrom, and magnetic lens means between said source and the targetfor defiecting and focusing the electron beam from its initial pathalong a curving path onto the surface of the target, said magnetic lensmeans including means outwardly spaced fro-m the target for establishinga first magnetic field of increasing strength in the direction of travelof the electron beam adjacent said source and in the initial path f theelectron beam, means outwardly spaced from the target for establishing asecond magnetic field of substantially constant strength in the curvingpath of the electron'beam between said first magnetic field and thetarget, and means for establishing a. 5

third magnetic field having a strength less than the strength target,said first magnetic field having lines of fiux of iucreasing curvatureextending in a direction generally rearwardly from the surface of thetarget, said first magnetic field extending-over about the firstone-fourth to about one-half of the distance of the curving path of theelectron beam, said secondmagnetic field extending between said firstmagnetic field and a point between about one-half to about two-thirdsKof the distance of the curving path of the electron beam, whereby anelectron beam generated by said source is guided from its initial pathalong a curving path and is focusedv onto the surface of the target.

7. An electron beam gun assembly in accordance with claim 6 wherein thethird magnetic field decreases in the direction of travel of theelectron beam.

' 8. An electron beam g-un assembly comprisingy a target, a source foremitting fa high power density electron beam in a direction away fromthe target along a'n initial path which diverges from a lineperpendicular to the surface of the target, said source being positionedbeneath the surface of the target and isolated therefrom, a pair of polepieces spaced outwardly from the target and above said source formagnetically defiecting the yelectron beam from its initial path along acurving path onto the target, means for energizing said pole pieces toestablish a magnetic field of increasing strength in the initial path ofthe electron beam, and a magnetic field of substantially constantstrength over a portion of -the curving path of the electron beambetween the magnetic field of increasing strength an-d the target.

9. An electron ibeam gun assembly comprising, a target, a shield belowand to the side of the target,'a source disposed beneath the shield foremitting a high density diverging electron beam in a direction away fromthe target along an initial path which diverges from a lineperpendicular to the surface of the target, a pair. of spaced polepieces spaced outwardly Afrom the target and said source, and means forenergizing said pole pieces to establish a first magnetic eld lofincreasing strength in the initial path of the electron beam', a secondmagnetic field of constant strength in the path of the electron beamleaving said first magnetic field of increasing strength, and a thirdmagnetic field of decreasing strength in the path of the electron beamleaving said lsecond magnetic field, said third-4 magnetic fieldextending nto the region above the target.

References Cited UNITED STATES PATENTS 3,068,309 12/1962 Hanks 219-1213,132,198 5/1964 Du Bois et al. 219-121 3,177,535 4/1965 Hanks 13-313,202,794 8/1965 Shrader et al. 219-121 3,204,096 8/1965 Anderson etfal. 219-121 5 RICHARD M. wooD, Primary Examiner.

W. D.v BROOKS, Assistant Examiner.

U.S. Cl. X.R. 13-11

